Autoclave for curing retreaded tires

ABSTRACT

An improved autoclave for curing retread tires includes a chamber with circulating air flow having turbulence generating devices located in a middle lengthwise portion of the chamber. The turbulence generating devices include apertures to guide air from a supply duct into the chamber, and/or wedge-shaped elements or fins mounted on the interior wall of the chamber to disrupt the air flow and cause turbulence.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] The invention relates to autoclaves for curing retreaded tires,that is, heat and pressure chambers for vulcanizing a material bonding anew tread to a used tire casing. More particularly, the invention isdirected to an improvement in the heat transfer characteristics of anautoclave operating with forced-air flow.

[0002] A type of autoclave in current use in the retread industryincludes a chamber having an elongated, cylindrical shape capable ofcontaining a group of tires for a curing process. The tires are encasedin flexible covers or envelopes, which are evacuated to remove air frombetween and around the new tread and tire casing. The tires in theenvelopes are suspended in the chamber to allow heated air to flow tothe outer circumference of the tire and tread. The air in the autoclaveis pressurized to force the new tread against the tire casing to achievea uniform, tight bond. The air is heated to a temperature sufficient toproduce the vulcanization reaction of the gum rubber layer interposedbetween the tread and casing.

[0003] A problem in the art exists in chambers that circulate airbetween the chamber and the heat source. The air returned to the chamberis heated to a sufficient temperature to vulcanize the entire group oftires in the chamber throughout the length of the chamber. Nonetheless,a “cold spot” can occur where the vulcanization reaction is lesseffective, between where the heated air enters the chamber from thesupply duct and where the air is taken into the heat source. Often, thecold spot is at approximately the midpoint of the chamber.

[0004] One attempt to resolve this problem involved mounting a cone atthe lead tire to deflect heated air entering the chamber from the supplyduct to flow around the outer circumferences of the tires, but theresult has not been satisfactory.

[0005] According to the invention, heat transfer to the tires in aforced air flow autoclave is improved by increasing the air turbulencein the chamber, in particular, in the middle portion of the chamber. Theinventors have theorized that the air flow through the chamber variesfrom a state of high turbulence where the air exits the return duct, toa smoother flow in the middle portion, followed by a more turbulent flowas the air is drawn from the chamber to the heat source. The smootherflow in the center portion is believed to be responsible for the lessefficient heat transfer in this section of the chamber. The invention isaccordingly directed to devices for disturbing the air flow in thechamber to increase turbulence.

[0006] A solution for generating turbulence in the air flow, accordingto the invention, is to direct or duct a portion of air from the supplyduct into the chamber at approximately the center of the flow path inthe chamber, which in a longitudinal chamber is approximately thelongitudinal midpoint. The ducted air is directed to flow counter orcross to the flow of air in the chamber, and accordingly, generatesturbulence in the chamber air flow. A simple ducting device is anaperture in the supply duct wall with a louver or flap extending intothe supply duct to guide air from the supply duct into the chamber. Theincreased turbulence in the air flowing through the chamber is believedto improve the heat transfer characteristics. In addition, the ductedair is hotter than the main air flow at the center, and accordingly, thelongitudinal temperature profile is made more uniform.

[0007] According to one embodiment of the invention, two apertures andlouvers are provided on the supply duct walls in opposed relationship.Preferably, the apertures and louvers are located at a common plane atabout the longitudinal midpoint of the chamber. However, other locationsin the middle portion of the chamber could be selected depending on theflow characteristics of a particular chamber or location of the coldspot in a particular chamber. Additional ducts could be placed inlongitudinally spaced relation in longer chambers to maintain a highlevel of turbulence.

[0008] Other devices for ducting air from the supply duct into thechamber are contemplated by the invention. For example, a nozzle or tubemounted to the supply duct wall and communicating with the supply ductcould effectively direct air into the chamber air flow to causeincreased turbulence.

[0009] Another embodiment of the invention includes placing turbulencegenerating devices on the interior wall of the middle portion of thechamber. Such devices could include wedges, fins, vanes, or otherelements to generate turbulence by disrupting the air flow patternand/or directing air toward the center of the chamber.

[0010] According to a preferred embodiment, the air-disturbing devices(apertures and/or turbulence generating devices) are located in themiddle third of the chamber, and more preferably, at the center of thechamber. By “middle portion” is meant the portion of the chamber inwhich a cold spot occurs, which can be determined directly bymeasurement of the temperature of the tires. Generally, this is themiddle third of the chamber; however, the exact location and extent ofthe middle portion may depend factors influencing the air flowcharacteristics, including the total length of the chamber, the spacebetween and around the tires, the tire tread design, and the air flowvelocity in the chamber. In longer chambers, the cold spot could occursomewhere in or take up the middle 50% of the chamber length, that is,beginning about 25% of the length from the air inlet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will be better understood by reference to thefollowing detailed description in conjunction with the appendeddrawings, in which:

[0012]FIG. 1 is a schematic view of an autoclave in longitudinal sectionin accordance with an embodiment of the invention;

[0013]FIG. 2 is a schematic view of the chamber of FIG. 1 in which thetires have been removed and including a turbulence generating aperturein accordance with the invention;

[0014]FIG. 3 is a schematic view of the chamber of FIG. 2 viewed from anopposite direction so that an interior of the supply duct is shown;

[0015]FIG. 4 is a view of a turbulence generating device in the form ofa nozzle;

[0016]FIG. 5 is a plan view of a turbulence generating device in theform of a wedge; and

[0017]FIG. 6 is a side view of the wedge shown in FIG. 4.

DETAILED DESCRIPTION

[0018]FIG. 1 is a simplified, schematic illustration of an autoclave 10for curing retreaded tires currently in common use in the industry. Theautoclave 10 includes an elongated chamber 20 providing a space forcontaining a plurality of tires 22 for a cure process. The tires 22 aresuspended from a rack or frame 24 so that heated air may circulatefreely about the tires. Typically the tires 22 are placed in envelopesor covers that are connected to a vacuum source (not shown) to evacuatethe interior of the envelope, especially the region of the new tread andtire crown, to eliminate air at the bonding site and to generate apressure differential between the chamber and the tread.

[0019] An autoclave of the type shown is used in the type of retreadprocess in which a cured tread and an uncured gum rubber layer areplaced on a prepared tire casing and the assembly is then heated underpressure to vulcanize the gum rubber, bonding the tread to the casing.

[0020] In the type of autoclave illustrated in FIG. 1, air is heated bya heat source 30, which can be, for example, a steam, water or oil toair heat exchanger. The heated air exits the heat source 30 at theoutlet 32 and passes through an supply duct 34 to a duct exit 26. Thesupply duct 34 of the type illustrated includes a sheet fastened to theinner wall of the chamber 20 to define a narrow passage. In the type ofautoclave 10 illustrated, two supply ducts 34 are provided in thechamber 20 on opposite sides of the frame 24. Heated air moving from theduct exit 26 reverses direction at the head of the chamber andcirculates through the chamber 20 to an inlet 28 of the heat source 30.A fan or other suitable air-moving device (not illustrated) moves theair through its circulation path. The invention is not limited to thisparticular type of air heating and ducting structure, and other suitablestructures could be substituted. For example, in another type ofautoclave, electric resistance heaters are mounted in an supply ductsimilar to that illustrated, and a fan forces air through the duct forheating and to return to the front of the chamber.

[0021] As mentioned above, a problem in a circulating air autoclave isthat a cold spot, or less efficient heat transfer area, can developsomewhere in the longitudinally middle portion 36 of the chamber 20. Thetire assemblies in this middle portion do not cure as quickly as thetires at the ends of the chamber 20.

[0022] The inventors have discovered that increasing the air turbulencein the chamber made the rate of cure of the tires more uniformthroughout the chamber. The inventors believe that air flow in themiddle portion 36 of the chamber 20 is less turbulent and more laminarthan the air flow near the supply duct exit 26 and near the heat sourceinlet 28. The movement of air from the supply duct exit 26 and to theheat source inlet 28 produces turbulence in the flow. The inventorsrealized that increasing the turbulence in the middle portion 36 wouldimprove the heat transfer to the tire assemblies in the middle portion.

[0023] As used herein, the middle portion 36 is meant to define a regionof the chamber encompassing the cold spot. This is believed to beapproximately the middle third of the air flow path for the typicalautoclave, but the exact location can depend on other factors. The coldspot in relatively shorter chambers could have a shorter extent and thecold spot in longer chambers could accordingly be somewhat longer. Inaddition, such things as air flow velocity, the space between or aroundthe tire assemblies, and the tread design could alter the location andextent of the cold spot. Those skilled in the art will recognize thatthe exact location and bounds of the cold spot can be determined bymeasuring the temperature in the tire assemblies arrayed along thelength of the chamber.

[0024]FIG. 2 illustrates an autoclave having a turbulence generatingdevice in accordance with the invention. The tires shown in FIG. 1 havebeen removed for clarity. An aperture 40 is formed in the wall of thesupply duct 34 at approximately the midpoint of the air flow path, whichin the chamber shown corresponds to the longitudinal midpoint of thechamber. The aperture 40 is formed with a flap, fin or louver (notvisible in this view) to guide air from the return duct 34 into thechamber 20. This ducted air, moving from left to right in the figure,enters the chamber 20 in counterflow and/or crossflow to the main airflow in the chamber, which moves from right to left in the figure. Theducted air disturbs the flow in the chamber, and causes or increases theturbulence in the flow, thus leading to an improvement in heat transfer.Another advantage is that the air entering through the aperture 40 ishotter than the main air flow at that point, which lost some heat to thetires. Thus, the ducted air adds heat energy to the main air flow at thechamber 20 center area making the temperature profile more uniform.

[0025]FIG. 3 illustrates the autoclave 10 of FIG. 2 from the oppositedirection, so that the interior of the supply duct 34 is shown. Theaperture 40 and louver 42 are on the supply duct 34 wall and the louver42 extends into the supply duct to help guide air (indicated by thearrow) into the chamber.

[0026] According to the invention, one or more apertures 40 and louvers42 can be formed in the wall of the supply duct 34 at locations whereincreased turbulence would be helpful. The inventors have found that twoapertures 40 and louvers 42 formed at approximately the longitudinalmidpoint and spaced circumferentially about the chamber to be effectivein making curing more uniform in a chamber sized for twenty-five tires.

[0027] Other devices for generating turbulence by ducting air from thesupply duct 34 to the interior of the chamber 20 could be used. Forexample, a tube or a nozzle structure 43, illustrated in FIG. 4, couldbe mounted on the supply duct 34 to guide air into the chamber 20. Atube or nozzle 43 could be used to better direct the ducted air to thetire assemblies, and an array of tubes or nozzles could providedisturbing flows in different directions in the chamber air flow. A hoodor scoop might be used to guide air into the nozzle or tube from theduct.

[0028] As would be recognized by those skilled in the art, the aperturesor nozzles could be used with any type of supply duct. Theaperture/louvers and nozzles are not mutually exclusive; a chamber couldbe equipped with both apertures and nozzles.

[0029] According to another embodiment of the invention,turbulence-generating devices are mounted on the interior of the chamberwall to disturb the main air flow. The turbulence generating devices arepositioned where the air flow begins to lose turbulence downstream ofthe supply duct exit 26. Typically, this would be about one-fourth toone-third of the air flow path, corresponding to one-fourth to one-thirdof the length of the illustrated chamber 20 measured from the supplyduct exit 26.

[0030]FIGS. 5 and 6 illustrate an example of a suitable turbulencegenerating device. FIG. 5 shows a wedge-shaped element 50 in plan viewpositioned to divide the air flow and generate a swirling region at itsdownstream end. FIG. 6 shows the wedge-shaped element in side view, theupper surface 52 acting as a ramp to guide air flow away from thechamber wall and across the air flow above the wedge 50. A plurality ofthese wedge-shaped elements positioned circumferentially around theinterior chamber wall could be used to disrupt the air flow and causeturbulence.

[0031] Other air-disturbing elements could be substituted, includingsimple flaps, fins, or vanes, as will be understood by those skilled inthe art.

[0032] The invention has been described in terms of preferred principlesand embodiments, however, substitutions and equivalents could beemployed without departing from the scope of the invention as defined bythe following claims.

What is claimed is:
 1. An autoclave for curing a tread strip to a tirecasing with heated, pressurized air, comprising: an elongated chamberhaving a frame for supporting tire casing and tread assemblies withinthe chamber; a heat exchanger for heating air; a supply duct defined bya wall having an inlet and an outlet at opposite ends, an aircirculating system for circulating air as a flow through the chamber,heat exchanger, and the supply duct; and air flow turbulence generatorsdisposed in the chamber in a main air flow.
 2. The autoclave as claimedin claim 1, wherein the air flow turbulence generators comprise at leastone aperture formed on the wall of the supply duct to guide air from theduct into the chamber in a direction disruptive to the main air flow inthe chamber.
 3. The autoclave as claimed in claim 2, wherein the atleast one aperture is formed at a midpoint of the main air flow in thechamber.
 4. The autoclave as claimed in claim 2, wherein the at leastone aperture includes a louver formed on the wall of the chamber toguide air from the supply duct.
 5. The autoclave as claimed in claim 2,wherein the apertures comprise at least one nozzle communicating withthe supply duct to guide air into the chamber in a direction disruptiveof the main air flow in the chamber.
 6. The autoclave as claimed inclaim 1, wherein the air flow turbulence generators comprise at leastone air deflector disposed on a wall defining an interior of thechamber.
 7. The autoclave as claimed in claim 6, wherein the at leastone deflector is disposed in a middle third portion of the air flow pathin the chamber.
 8. The autoclave as claimed in claim 6, wherein the atleast one deflector is a fin.
 9. The autoclave as claimed in claim 6,wherein the at least one deflector is a wedge-shaped element.
 10. Theautoclave as claimed in claim 6, wherein the at least one deflector is avane.